The present invention relates generally to time domain reflectometers and, more particularly, to a method of minimizing signal errors and anomalies.
With the ever-increasing number of communication and transmission cables being utilized throughout the world, it is desirable that anomalies such as faults, partial discharges, cable damage, and splices on communication and power transmission cables be located without the necessity of physical tracing and inspection. A Time Domain Reflectometer (TDR) can be used to analyze a cable for anomalies or changes in cable impedance in order to locate such anomalies. A typical TDR transmits a pulse of electrical energy onto cables that include two conductors separated by a dielectric material. When the pulse encounters a change in the impedance of the cable, part of the pulse""s energy is reflected back toward the TDR. The amplitude and polarity of this reflection is proportional to the change in impedance. Such reflections are usually displayed in graphical form on the screen of a typical TDR whereby a technician can interpret the results and locate specific cable anomalies.
In the past, a technician""s ability to interpret a displayed waveform has been limited because of a TDR""s inability to provide high quality information. Information correlating to the portion of cable located closest to the TDR is of higher quality than that portion of the cable remotely located from the TDR. This is because the reflection signal degrades as the length increases. As a result, a waveform decreases in accuracy as the distance between that portion of the cable being measured and the TDR increases. Currently, there are several available solutions to overcome waveform degradation. One such solution is to locate the TDR at both ends of the cable being analyzed. This is undesirable because the technician would have to manually compare the two waveforms and make a calculation to determine the location of objects of interest, such as the location and determination of anomalies.
Another solution is to connect a signal wire to each end of the cable and simultaneously measure the refection wave. The TDR would then be able to process the two signals to better pinpoint anomalies. This is undesirable because a great length of test leads are necessary to measure two ends of a long portion of cable simultaneously with a single TDR. Additional problems arise when a standard three-phase power cable is analyzed and only one phase at a time can be recorded. This results in potential human comparison errors when deciphers splice and fault locations. In multiple conductor cables, this problem is even more evident.
Another problem that has arisen with the use of a currently available TDR, is cable medium with changing segment impedance. Often times, a cable contains several segments of different conductive mediums spliced together to form one cohesive length of cable. The reason segmented cables exist is due to portions of the length having been replaced with different conductor materials because of damage to the cable or the need replace particular sections of the cable with a different medium. A change in the medium will affect the impedance because of small differences in the cable""s, manufactured geometry or materials thereby affecting signal""s velocity of propagation (VOP). This results in inaccurate information of anomalies further down the conductor. Other factors may affect the VOP as well, such as a change in the dielectric material that separates conductors within a cable. Water flooding in the interior of cables that use air as part of the dielectric separation of conductors has been a particular problem that affects the VOP of a signal from a TDR.
Thus, there exists a need for graphically representing information collected from a device propagating a signal along the length of a cable.
One embodiment of the present invention provides a method, apparatus and computer-readable medium for improving the quality and accuracy of information collected by propagating a signal along a length of cable in order to pinpoint specific anomalies along the length of cable. This embodiment improves quality and accuracy by displaying multiple waves simultaneously and combining several steps of signal processing to raw data collected by a TDR. The signal processing steps include: signal data collection, wave reversal, wave shifting, multi-wave display, segmented velocity of propagation, multi cursor, and wet cable calculator.
Using the various embodiments of the present invention in conjunction with a TDR, a technician can record, modify, and display several waveforms corresponding to specific cables from either end of specific cables and process the information collected and recorded at a later time. Specifically, a technician can take a set of two recorded waveforms that are collected from the same cable and compare the waveforms to determine the location of anomalies. If the two waveforms are recorded from opposite ends of the cable, then wave reversal can be used to process the waveforms in order to produce a more accurate representation of the location of anomalies along the cable.
As a non-limiting example, if the two waveforms are recorded from two different points on a cable in the same direction of propagation, then wave shifting can be used to process the waveforms in order to produce a more accurate representation of the location of anomalies along the cable.
In order to more accurately decipher the location of anomalies along a set of conductors, multiple waveforms can be displayed simultaneously. A technician can easily pinpoint the location of particular anomalies, such as three phase faults or severed cables, by analyzing several waveforms simultaneously.
Additionally, in another embodiment of the present invention, the accuracy of locating anomalies can be improved if the technician is aware of segments of differing mediums along the length of cable. By identifying the particular medium of the segment on which the signal is propagating, the TDR can compensate for a change in VOP which would affect the accuracy of the anomaly""s actual location. A typical TDR will measure the time interval between two cursors that can be manually or automatically positioned. Because of this limitation of two cursors, several segments had to be analyzed separately. However, the various embodiments of the present invention are capable of employing several cursors simultaneously to analyze the entire length of cable with several different mediums, and subsequently each with a differing VOP.
Finally, in still yet another embodiment of the present invention, the calculation of the total length of water affecting the impedance of a cable is now possible. A technician knowing this information is able to adjust the signal processing in order to take this condition into account prior to identifying anomalies and their respective locations along the cable. This embodiment also improves the accuracy of locating anomalies.
A method, apparatus, and computer-readable medium capable of performing actions generally consistent with the foregoing data acquisition and signal processing for determining the location of anomalies along a cable is presented in further detail below.